Conventional electrical installations for incandescent lamps usually miss protection devices for the filaments of the incandescent lamps. They will upon switching on the light be subjected to a surge current which considerably exceeds the normal operation current. The size of the surge current depends on where the switching on the current takes place by time along the sinus curve of the supply current and can amount to fourteen times the normal operation current. Even though the switching on takes place in the zero-point of the sinus curve, the operating current will be exceeded with a factor of four in connection with conventional installations. This depends on the fact that the resistance of the filament is substantially proportional to the absolute temperature. The resistance of a cold filament is ten times less than the filament at normal operating temperature, involving said surge current, which strongly limits the life time of conventional incandescent lamps. In the filament a gasous process takes place resulting in a consumption of the material of the filament, usually containing tungsten, until a brakage occurs. It has been stated that this process is accelerated in connection with switching on by means of conventional technique because of said surge current.
Attempts have been made to prolong the life time by protecting incandescent lamps against such surge currents by means of a thermistor with a negative temperature coefficent which is connected in series with the filament. Such a proposal is apparent from the U.S. Pat. No. 3,975,658. However, this known solution is provided with disadvantages. A thermistor consumes power which reduces the light with for example up to 8%. Furthermore, it has appeared that thermistors per se will be heated relatively fast and therefore the operating current will be exceeded with 3-4 times despite this known protection device. The disadvantageous surge current in connection with switching on will consequently not be limited but only reduced and will furthermore vary in size depending on where along the sinus curve the switching on occurs.
Consequently it can be stated that hitherto known solutions have not in a satisfactory manner solved the problem with the limited life time of incandescent lamps.